A circle with radius $9$ has a sector with a $180^\circ$ central angle. What is the area of the sector? {81\pi} \color{#9D38BD}{180^\circ} {\dfrac{81}{2}\pi} {9}
Explanation: First, calculate the area of the whole circle. Then the area of the sector is some fraction of the whole circle's area. $A_c = \pi r^2$ $A_c = \pi (9)^2$ $A_c = 81\pi$ The ratio between the sector's central angle $\theta$ and $360^\circ$ is equal to the ratio between the sector's area, $A_s$ , and the whole circle's area, $A_c$ $\dfrac{\theta}{360^\circ} = \dfrac{A_s}{A_c}$ $\dfrac{180^\circ}{360^\circ} = \dfrac{A_s}{81\pi}$ $\dfrac{1}{2} = \dfrac{A_s}{81\pi}$ $\dfrac{1}{2} \times 81\pi = A_s$ $\dfrac{81}{2}\pi = A_s$